1. Technical Field
This disclosure relates to an electrophotographic photoconductor. In addition, this disclosure relates to an image forming method, an image forming apparatus, and a process cartridge, which use the electrophotographic photoconductor.
2. Description of the Related Art
In an electrophotographic image forming method using an electrophotographic image forming apparatus, an image is formed by performing processes such as a charging process, an irradiating process, a developing process, and a transferring process on an electrophotographic photoconductor (hereinafter referred to as a photoconductor) serving as an electrostatic image carrier or an image carrier. Recently, organic photoconductors using an organic photosensitive material are broadly used as the photoconductor because organic photosensitive materials have a good combination of flexibility, heat stability, and film formability.
Among various organic photoconductors, functionally separated multi-layer photoconductors including an electroconductive substrate, and a photosensitive layer, which is located on the electroconductive substrate and in which a charge generation layer including a charge generating material and a charge transport layer including a charge transport material are laminated, prevail recently. Among various functionally separated multi-layer photoconductors, a number of negatively-chargeable photoconductors, which include a charge generation layer which is a deposited layer of an organic pigment serving as a charge generation material or a layer including a resin and an organic pigment dispersed in the resin, and a charge transport layer in which an organic low molecular weight compound serving as a charge transport material is dispersed in a resin, have been proposed recently. In addition, a technique, in which an intermediate layer (which is sometimes referred to as an undercoat layer) is formed between an electroconductive substrate and a photoconductor to prevent injection of a charge into the photosensitive layer from the electroconductive substrate, is proposed.
Recently, electrophotographic image forming apparatus have been improved to produce high resolution full color images at a high speed, and therefore a need exists for a photoconductor having a good combination of durability and stability. In this regard, when organic photoconductors are used for the current electrophotographic image forming process in which charging and discharging are repeatedly performed, properties of the organic materials constituting the organic photoconductors change due to the electrostatic load on the organic materials, thereby causing a problem in that the electrophotographic properties of the photoconductors deteriorate due to formation of a charge trap in the photosensitive layer, and change of the charging property of the organic materials.
Particularly, when the charging property of organic photoconductors deteriorates due to alteration of the photoconductors, the quality of images produced by the photoconductors seriously deteriorates. Specifically, in this case, image quality problems such that image density decreases; background of images is soiled with toner (hereinafter sometimes referred to as background fog); and the image quality changes when images are continuously produced are caused.
One of the reasons for deterioration of the charging property of organic photoconductors is considered to be deterioration of the intermediate layer of the photoconductors. In general, such an intermediate layer is required to have both a charge injection preventing function to prevent injection of a charge into a photosensitive layer from an electroconductive substrate, and a charge transport function to transport a charge generated in the photosensitive layer to the electroconductive substrate. However, these functions typically establish a trade-off relationship, and in addition organic materials constituting such an intermediate layer tend to deteriorate due to repeated application of electrostatic load thereon. Therefore, it is hard for the intermediate layer to maintain a good combination of the above-mentioned functions over a long period of time.
In attempting to impart a good combination of the functions to an intermediate layer, techniques in that a silane coupling agent including an amino group is used to enhance the charge injection preventing function; and techniques in that an additive such as an electron transport material or an acceptor compound is included in the intermediate layer have been proposed.
The second mentioned techniques include a technique in that an undercoat layer including a particulate metal oxide to which an electron acceptor compound (such as hydroxyanthraquinone compounds and aminohydroxyanthraquinone compounds) is adhered is formed on an electroconductive substrate.
Recently, electrophotographic image forming apparatus have been downsized, and therefore photoconductors used therefor have also be downsized. In addition, a need exists for an image forming apparatus which can produce images at a high speed while being maintenance free. Therefore, a need exists for a photoconductor having good durability.
However, the life of photoconductor also depends on electric properties of the photoconductor, i.e. a charging property in a dark place, and a charge decaying property such that charges on the photoconductor rapidly decay in an irradiating process. Therefore, it is important for photoconductor to maintain a good combination of the charging property and the charge decaying property over a long period of time to prolong the life thereof.
As mentioned above, the intermediate layer is required to maintain both the charge injection preventing function and the charge transport function over a long period of time. However, in general, the number of traps in the intermediate layer, which inhibit flow of charges through the layer, increases when the photoconductor is repeatedly used over a long period of time, thereby increasing the potential of an irradiated portion of the photoconductor. In this case, the contrast between the potential of a non-irradiated portion (i.e., a dark portion) of the photoconductor and the potential of the irradiated portion decreases, thereby decreasing the image density, resulting in shortening of the life of the photoconductor.
Since the outermost layer of organic photoconductor typically includes a low molecular weight charge transport material and an inert polymer as main components, the outermost layer is typically soft. Therefore, when such an organic photoconductor is repeatedly subjected to electrophotographic processes (such as charging, irradiating, developing, transferring, cleaning and discharging processes), the organic photoconductor tends to cause problems such that the outermost layer is easily abraded due to mechanical loads applied in the developing process and the cleaning process.
When the photoconductor is excessively abraded, the potential of an irradiated portion of the photoconductor increases, resulting in shortening of the life of the photoconductor. Therefore, photoconductor is required to have small abrasion loss to prevent increase of the potential of an irradiated portion.
Further, in order to produce high quality images, the particle diameter of toner becomes smaller and smaller. In this regard, it becomes hard for a cleaning blade to remove such a toner having a small particle diameter from a photoconductor, and therefore, recently, the hardness (rubber hardness) and contact pressure of the cleaning blade are increased, resulting in acceleration of abrasion of the photoconductor. Abrasion of photoconductor deteriorates the photosensitivity and electric properties such as charging property of the photoconductor, thereby forming low density images and abnormal images such as background fog.
In addition, when a scratch (i.e., local abrasion) is formed on the surface of the photoconductor, the surface of the photoconductor is defectively cleaned, thereby forming an abnormal streak image. Therefore, not only abrasion but also such a scratch shortens the life of the photoconductor.
In attempting to enhance the abrasion resistance of photoconductor (photosensitive layer), the following technics (1) to (3) have been proposed.
(1) A curable binder resin is used for the outermost layer;
(2) A charge transport polymer is used for the outermost layer; and
(3) An inorganic filler is included in the outermost layer.
In addition, a photoconductor having an outermost layer including an electroconductive filler is proposed to improve the electric property of the photoconductor.
Further, there is a proposal such that a charge transport layer is formed by curing a monomer having a carbon-carbon double bond, a charge transport material having a carbon-carbon double bond, and a binder resin. The binder resin includes a resin which has a carbon-carbon double bond and which can be reacted with the charge transport material, and another resin which has no carbon-carbon double bond and which cannot be reacted with the charge transport material.
Furthermore, there is a proposal for a photosensitive layer including a cured material obtained by curing a positive hole transport compound having two or more chain polymerizable functional groups in one molecule thereof.